DO macro and functional programming

Hi folks,

I added a GEP-23 and GEP-24.

GEP-23 is for a DO macro and some optional type checkers that can be
used with it.

https://groovy.apache.org/wiki/GEP-23

GEP-24 is a placeholder for Groovy 7 if we ever wanted to provide Try
Monad like support. I suspect it might change a little before we
action it

Some of the design decisions around the type checkers in GEP-23 may
not seem obvious, so I also created a blog post on some of the recent
functional additions (currently including GEP-23 in its proposed
form):

https://groovy.apache.org/blog/groovy6-functional

Feedback welcome.

Thanks, Paul.